This invention relates to a radio frequency ("RF") electronic ballast with power factor correction for induction-coupled gas discharge lamps, and particularly to an electronic ballast for such lamps which integrates the power factor correction function with the ballast's inverter.
An induction lamp (electrodeless fluorescent lamp) may be operated at radio frequencies, e.g., 2.65 MHz. As shown in FIG. 1, a conventional electronic ballast 10 for an induction lamp 12 powered from the AC line may include a bridge rectifier 14, a DC filter capacitor Cb and an RF resonant inverter 16 coupled to an output coil La. The output coil La, which is excited by RF energy from the inverter 16, is inductively coupled to the lamp 12. When power factor correction is desired, a power factor correction stage 18 is coupled to the output of the rectifier 14, and with the filter capacitor C.sub.B provides a power factor corrected DC voltage to the inverter 16. This approach uses two-stages, the power factor correction stage 18 to provide a DC output voltage and the inverter stage 16 to provide the RF energy to the output coil La.
The ballast 10 includes conventional fast reverse recovery, silicon isolating diodes D1 and D2, an input or boost inductor Li (e.g., PC40 EF25/11), and an inductor Lf and capacitor Cf which form a low pass input filter. The inverter 16, which is also conventional, includes two switches S1 and S2 connected as a half bridge resonant inverter, and a resonant tank 20 comprising inductor Lr, and capacitors Cs and Cp. The power factor correction stage 18, which is conventional, includes a switch S3. A conventional control circuit, not shown, controls the inverter switches S1 and S2, and another conventional control circuit, not shown, controls the power factor correction switch S3.
Energy has to be processed twice in the ballast 10, once by the power factor correction stage 18 and once by the inverter 16. Therefore, power conversion efficiency could be low. Additionally, the number of components required for the separate power factor correction and RF energy generation stages is high, which increases cost.
U.S. Pat. No. 5,446,350 discloses an RF electronic ballast for an electrodeless induction lamp including an RF resonant tank circuit with an additional capacitor in the resonant tank circuit, which the patent states reduces the phase angle presented to the ballast, and thereby lowers the sensitivity of the ballast to component and lamp variations.
The following documents disclose the use of feedback in electronic ballasts which are direct or transformer coupled to the lamps (and not electrodeless) U.S. Pat. No. 5,404,082, European Patent Publications EP 395,776 and EP 726,696.
None of the documents cited above employs feedback in an RF electronic ballast for an electrodeless induction lamp to achieve power factor correction.